1. Field of the Invention
The present invention relates to a coil to be used suitable as an inductor or transformer for various electric devices, and more particularly to a thin-type coil advantageously suitable for contactless power transmission (contact-free power transmission).
2. Description of the Related Art
The technology of contactless power transmission designed so that power exchange is performed in a contactless (contact-free) manner by electromagnetic coupling of a coil in a transmitter and a coil in a receiver has found use in cellular phones, portable information terminal devices, and home electric devices (see Japanese Laid-Open Patent Publication No. 2008-172872 and Japanese Laid-Open Patent Publication No. 2005-6440 below).
In a large number of electric devices using such contactless power transmission, miniaturization and thickness reduction are strongly required for devices on both the transmission side and the reception side, and thin-type air-core coils that can easy fulfill such requirements are often used in the coils employed in the devices. Further, since the terminals connected to the end portions of the coil are typically disposed on the outer circumferential portion of the coil, both the one side and the other side of the winding wire of the coil should be together drawn forth to the outer circumferential side of the coil.
A method by which the other side of the winding wire is spirally wound from the inner circumferential side towards the outer circumferential side, while leaving the one side of the winding wire on the inner circumferential side, and then, after the winding is completed, drawing forth the one side of the winding wire from the inner circumferential side towards the outer circumferential side by dragging the one side of the winding wire over the coil flat surface (referred to hereinbelow as the “usual winding method”) is generally known as a coil winding method. When the usual winding method is used, the winding wire should be bent when one side of the winding wire is drawn forth from the inner circumferential side onto the coil flat surface. Therefore, extra force can be easily applied to the winding (in particular, in the case of a single wire winding of a large diameter), and the drawing line portion of the winding increases in thickness by the winding wire diameter (see Japanese Laid-Open Patent Publication No. 2008-172872, FIGS. 27 and 28).
A winding method (referred to hereinbelow as “α winding method”) in which a winding shaft is set close to the center at both ends of the winding and the one side and the other side of the winding wire of the coil are spirally wound in mutually opposite directions is also known as described in Japanese Patent No. 4321054 below. With the α winding method, the one side and the other side of the winding wire can be drawn forth to the outer circumferential side, without applying an unnecessary force to the winding.
In a contactless power transmission system, when the coil diameter of the transmitter and the coil diameter of the receiver are equal to one another, electromagnetic coupling between the coils is good and therefore power can be transmitted with good efficiency. In the typical conventional contactless power transmission system, only one coil having a diameter equal to the coil diameter of the receiver is provided in the transmitter. Therefore, efficient power transmission is difficult to realize with respect to a receiver having a coil of a different diameter.
To resolve this problem, in the contactless power transmission system described in Japanese Laid-Open Patent Publication No. 2005-6440, it is suggested to provide a plurality of coils arranged side by side inside a transmitter (charger) to adapt to receivers (cellular phones) of a plurality of types that differ in specifications. Where the diameters of the plurality of coils in the transmitter are made to differ from one another, it is easy to transmit power with good efficiency to receivers of a plurality of types that differ from one another in a coil diameter. However, the problem is that since a space should be ensured to accommodate a plurality of coils side by side, the transmitter is increased in size.
In order to reduce the space for accommodating the plurality of coils, it is possible to consider a configuration in which a large-diameter coil and a small-diameter coil are piled up on the same shaft, and the usage mode is switched between the large-diameter coil and small-diameter coil correspondingly to the coil diameter of the other device. With such a configuration, the space in the coil diameter direction can be reduced by comparison with the case in which the large-diameter coil and small-diameter coil are arranged side by side on the same plane. However, the problem is that because the coils should be piled up on the same shaft, a larger space is required in the direction of the winding shaft axis (height direction).
Accordingly, it would be convenient to have a coil such that makes it possible to change the diameter of the region that is used as a coil correspondingly to the size of the coil of the other device. For example, a configuration can be considered in which a winding wire wound to a small diameter is disposed inside (inner diameter portion) of an annularly wound winding wire and the configuration can be switched between a mode in which only the small-diameter winding is used as a coil and a mode in which the small-diameter winding and the annular winding are used together as one large-diameter coil correspondingly to the coil diameter in the other device. With such a configuration the thickness of one coil in the direction of the winding shaft axis can be decreased, while adapting to the difference in coil size in the other device. However, the problem is that because a space should be provided between the annular winding and the small-diameter winding in order to combine the two windings together, the diametrical size of one coil should be rather increased.